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George R. Anderson and J. Paul Devlin
Electron Oscillation Effects in the Vibrational Spectra of Tetracyanoquinodimethane Ion Radical Salts George R. Anderson* Department of Chemistry, Universlty of Minnesota, Mlnneapolls, Minnesota 55455
and J. Paul Devlin Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74074 (Received January 3 1, 1975) Publication costs assisted by the National Science Foundation
Polarized infrared reflection spectra of crystalline potassium tetracyanoquinodimethane (KTCNQ) are presented and vibrational assignments are discussed. Five bands in the 700-2500-~m-~spectral range are reported to originate from totally symmetric (Ag) molecular vibrations of the TCNQ- anion, based on their intense out-of-plane polarization. A vibronically based charge oscillation, that has previously been recognized for charge-transfer complexes and certain TCNE salts, is thought to be the principal cause for the breakdown of the vibrational selection rules. These results seem to necessitate a drastic reassignment of the vibrational spectrum of this and possibly other radical anion salts. There have been several recent studies of the vibrational spectra of radical anion ~ a l t s l -particularly ~ of tetracyanoethylene2 (TCNE) and tetracyanoquinodimethane (TCNQ).3,4The Raman spectra of these strongly colored compounds have, in each case, been marked by the resonant nature of the scattering. This character has been well demonstrated in a wavelength-dependent study of the scattering from TCNQ salts.4 Although the Raman spectra have, in all cases, been for polycrystalline samples, the additional data provided by the wavelength dependence of the scattering, as related to the positions of the electronic absorption bands, have made possible a reliable assignment of the Raman bands. The assignment of the infrared spectra of radical salts in general, and TCNQ salts in particular, is a subject of some disagreement. Through a comparison of the infrared and Raman spectra of M+TCNE- salts and a qualitative application of the Ferguson-Matsen-Friederich-Person (FMFP) electron oscillation theory for charge-transfer syst e m ~ Hinkel ,~ and Devlin showed that each of the dominant features in the TCNE- infrared spectrum is based on the A, (infrared inactive) modes of the parent molecule.2 This implies an infrared activation of these modes through a vibronic interaction mechanism that has been referred to as an “electron oscillation”. The key characteristic of such an activation is that the transition dipole is along a coordinate joining donor (M+) and acceptor (TCNE-) species regardless of the direction of the atomic displacements for the mode. The M+TCNE- infrared assignment seems firmly established. However, the published infrared data for the TCNQ slats have been interpreted with little regard for the possible (dominant) role of this electron oscillation m e ~ h a n i s m . ~Thus, , ~ , ~ each of the strong infrared features has been assigned to a Bl,, Bzu, or Bsu TCNQ- mode, and Girlando et al. have stated that there is no evidence that vibronic effects are i m p ~ r t a n t . ~ Since the TCNQ- infrared assignments have been employed in subsequent normal coordinate analyses and the computed force constants have been used to argue for the effect of the radical electron on the bonding in the TCNQ
* Address correspondence to Bowdoin College, Brunswick, Maine 04011. The Journal of Physical Chemistry, Vol. 79, No. 11, 1975
a n i ~ n ,the ~ ? validity ~ of the assumption that vibronic effects are unimportant needs examination (particularly in view of the earlier speculation that the dominance of vibronic effects in the infrared spectra of radical anion salts is likely a general phenomenon2). Considering the crystal structure for K+TCNQ-, the anions are stacked approximately plane-to-plane in parallel columns along the needle ( a ) axis of the crystal.819The TCNQ- planes are inclined about 10’ from normal to the a axis. Alternating spacings between neighboring TCNQ molecules down the stack suggest that not all intermolecular interactions are equivalent and that pair-wise interactions (of dimers) may be important from a spectroscopic point of view. Indeed, the near-ir electronic transition in KTCNQ at ca. 10,000 cm-l is by all evidence a dimer-type charge transfer band and polarized parallel to the a a x k 8 Thus, it is a simple matter to design an infrared experiment to determine the importance of the electron-oscillation mechanism. The infrared active fundamental modes for neutral TCNQ in the 900-4000-~m-~range are in-plane in nature so that, ignoring any vibronic effects, the transition dipole for these modes should be perpendicular to the crystallographic a axis. On the other hand, the strong interactions of the anion species with each other and with the neighboring cations are out of the molecular planes so any vibronically based electron oscillation should have an outof-plane character. Thus, the determination of the transition dipole directions by polarized infrared measurements on single crystals of K+TCNQ- should immediately indicate the role that vibronic effects play in the TCNQ- salt spectra and either affirm the suggested vibrational assignm e n t ~ ~or, necessitate ~,~ a radically different interpretation. Actually the requisite infrared measurements and interpretation were made and reported several years ago,1° but not in the open literature, so a brief review of the results and their implication is in order. Because of the optical density of the salts, the single crystal infrared polarization study was made by reflection, rather than absorption, but the resultant curves, Figure 1, are conclusive in their import. These spectra show that the transition moments for the intense bands 2, 3, 6, 8, 10, and 11 are parallel to the needle axis. Of these, only band 10, for the C-H out-of-
Vibrational Spectra of TCNQ Ion Radical Salts
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Figure 1. Infrared spectra of crystalline KTCNQ ion-radical salt: (A) absorption spectrum using the KBr pellet technique; (B) room temperature polarized reflection spectra at near-normal incidence where the solid line refers to ir polarization with the E vector parallel to the a crystallographic (stacking)axis while t h e broken line refers to polarization perpendicular to this axis and approximately parallel to the molecular planes.
plane mode, would normally have such a polarization. The other five bands, which dominate the TCNQ- infrared spectrum, cannot result from normal infrared active inplane modes. The latter, however, are likely sources for bands 1, 4,5, and 9 which have the expected polarization. The data show that at least five planar modes are accompanied by a strong charge oscillation perpendicular to the direction of atomic motion but in the direction of strong intermolecule interactions. This is characteristic of the FMFP charge oscillation for charge-transfer system^.^ Since the FMFP activation mechanism is normally strongest for the totally symmetric (A,) modes,ll the results are most simply interpreted by assigning features 2, 3,6,8, and 11to A, modes of the isolated TCNQ- ion. The instrument used to record the near-normal incident reflection spectra has been described previously.lZ The KTCNQ crystals (needles) were grown from CH3CN solution. The reflectance surface was composed of six oriented crystals in a mosaic rectangle, 2.0 X 0.63 mm dimensions. Reflection studies a t 77’ and 4.2’K show no appreciable changes in polarization or band intensities. The basis for ignoring vibronic effects in previous M+TCNQ- studies was the lack of coincidence between infrared and Raman band position^.^ A comparison of the features of Figure 1 with the published Raman spectra3p4 show that only the infrared bands at 825 and 1512 cm-l
have no Raman counterpart within 40 cm-l. The 825-cm-l feature is for the C-H out-of-plane mode while Figure 1 shows that the 1512-cm-l band results from normal inplane infrared activity, so neither feature should have an intense Raman counterpart. The small (less than 40 cm-l) difference between the infrared features 2, 3, 6, 7, and 11 and their Raman counterparts, though large for factorgroup splittings, are smaller than the factor-group splittings reported for KTCNE2 and similar in magnitude to such splittings reported for the KTCNQ Raman s p e ~ t r u m . ~ The point is that, since there are eight TCNQ- units per unit cell,9 each TCNQ- fundamental is the basis for eight factor-group components. The Raman active componenis based on the A, molecular anion modes will have a somewhat different frequency than the infrared components based on the same molecular modes. I t seems unnecessary to tabulate a new K+TCNQ- vibrational assignment since the outgrowth of this analysis is simply stated: the intense infrared bands a t 2175, 1583, 1343,1183, and 719 cm-l should be attributed t o molecular anion Ag modes (YZ, v3, u4, v5, and v7, respectively) rather than as previously a ~ s i g n e d .We ~ , ~would also like to point out the noticeably greater line width of these absorption bands. Compared with bands 1, 4,5, and 9, these electronoscillation modes are found to be 2-3 times broader than their simple molecular counterparts. The origin of this The Journal of Physical Chemistry, Vol. 79, No. 7 1, 1975
Douglas C. McCain
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added line width is not clearly understood. Other lower frequency modes, not within the range of this study, have similarly been assigned incorrectly. Since the infrared absorption and polarized reflection data for Cs2TCNQ3 are similar to that for KTCNQ, the interpretation here is presumed to extend to that salt as well. In fact these.results, combined with those for M+TCNE- salts? strongly suggest that vibronic effects must be routinely considered in analyzing the vibrational spectra of radical anion salts. If either ion pairing or dimerization is likely, this view must be extended to solutions of ion radical salts. Only in the absence of strong association with a second species are such FMFP-type activations of symmetric modes improbable. Acknowledgment. We wish to acknowledge the Central Research Department, E. I. du Pont de Nemours and Company, where the experimental work was carried out by one of the authors (G.R.A.). Also, we wish to acknowledge the help of Dr. G.J. Sloan in growing crystals for this study and of Dr. C.J. Fritchie for his work and discussions on the crystal structure of KTCNQ. Dr. R.E. Merrifield is ac-
knowledged for his long-term interest and useful discussions regarding the TCNQ salts. References a n d Notes P. C. Li, J. P. Devlin, and H. A. Pohl, J. Phys. Chem., 76, 1026 (1972). J. J. Hlnkel and J. P. Devlin, J. Chem. Phys., 58, 4750 (1973). A. Glrlando, R. Bozio, and C. Pecile, Chem. Phys. Lett.. 25, 409 (1974). C. Chi and E. R. Nlxon, Spectrochim. Acta, in Dress. H. B. Friedrich and W. B. Person, J. Chem. Phys., 44, 2161 (1966); E. E. Ferguson and F. A. Matson, ibid. 29, 105 (1958). NOTEADDEDIN PROOF: -One reviewer cited an omission of important re:ent references in ref 5. These are: R. S. Mulliken and W. B. Person, Molecular Complexes”, Wiley, New York, N.Y., 1969; W. B. Person, in “Spectroscopy and Structure in Molecular Complexes”. J. Yarwood, Ed.. Plenum Press, London, 1973, Chapter 1. (6) M. G. Kaplunov, T. P. Panova, E. B. Yagubskii, and Y. G. Borod’ko, Zh. Strukt. Kbim., 13,440 (1972). (7) A. Girlando, L. Morelll, and C. Pecile, Chem. Phys. Lett., 22, 553 (1973). (8) G. R. Anderson and C. J. Fritchie, Jr., Paper 111, Second National Meeting of the Society of Applied Spectroscopy, San Diego, Calif., 1963. (9) R. P. Shibaeva and L. 0. Atovmyan, Zh. Strukt. Khim., 13, 546 (1972). (IO) G. R. Anderson and R. L. McNeely, Paper R5, Symposium on Molecular Structure and Spectroscopy, Ohio State University, June 1963. (11) E. E. Ferguson, J. Chim. Phys., 61, 257 (1964). (12) G. R. Anderson, J. Chem. Phys., 47, 3853 (1967). (1) (2) (3) (4) (5)
Inductive Effects and Franck-Condon Shifts in the Visible Spectra of Substituted Chromate Ions Douglas C. McCain Department of Chemistry, University of Southern Mississippi, Hattiesburg, Mississippi 3940 1 (Received October 9, 1974) Publication costs assisted by the University of Southern Mississippi
Ligand shifts in the visible range absorption spectra of substituted chromate ions, Cr03Ln-, support a postulated donor-acceptor interaction between the ligand and Cr03. A good correlation is found between Taft’s inductive substituent coefficient, CI,and the energy of the charge transfer band origin. Poor correlations between UI and absorption maxima are explained in terms of the Franck-Condon effect.
Introduction One of the first steps in a chromate oxidation involves formation of a substituted chromate ion,l Cr03Ln-, where the ligand Ln- may be an organic2 or an inorganic3 Lewis base. Especially noteworthy are the chromate esters, Cr03(0R)-, well known as intermediates in the oxidation of a l c ~ h o l s . ~Other , ~ chromates which have been shown to participate in oxidation reactions include3Bp7 Cr03C1- and Cr03(SCN)-. Substituted chromates typically display three distinct bands in their visible and near-ultraviolet ~ p e c t r aFigure .~ 1 shows a weak band at about 450 nm and the edge of a very strong, broad band which extends beyond 300 nm. The intervening band has been identified6,8s9with oxygento-chromium charge transfer. It exhibits a partially resolved vibrational progression due to excitation of a symmetric stretching mode in the Cr03 group. Such spectra are often seen during chromate oxidations and have been used to identify reaction intermediates. This study examines the The Journal of Physical Chemistry, Vol. 79, No. 11, 1975
effects of various ligands on charge transfer spectra. Aprotic solvents were chosen for optimum resolution; vibrational structure is often unresolved in water solutions. Experimental Section Reagent grade chemicals were used without further purification. Tetra-n- butylammonium salts were prepared from [(n-Bu)4N]C104 and KBr or KSCN. Traces of perchlorate in the product present no problem because clodis not expected to form a stable complex with Cr03. Tetraphenylarsonium salts were made in a similar way from NH4F, KBr, and KSCN. Solutions containing the tertiary butoxide ion, (Ot-Bu)-, were obtained by adding K(0t-Bu) to a solution of [(n-Bu)4N]C104. Hexakis(methy1isonitrile)iron(II) hydroxide was made from [Fe(CNCH3)6](HS04)2 and Ba(OH)2. [Fe(CNCH3)6](HSO& and [Fe(CNCH&]C12 were prepared by a standard method.1° The solvents used in this study were benzene, chlorobenzene, and dichloromethane; all were dried over Na2S04.
